1. Field of the Invention
The present invention relates to a method of strengthening the surface of alumina-zirconia composites with MoO.sub.2 as an accelerant for phase transformation of ZrO.sub.2 from tetragonal to monoclinic. More particularly, the present invention relates to a method of using MoO.sub.2 for accelerating the phase transformation of zirconia from tetragonal to monoclinic to strengthen the surface of alumina-zirconia composites.
2. Description of the Prior Art
Zirconia (ZrO.sub.2) is a ceramic material which is widely used as structural parts, chemical sensors, and so on, attributing to its unique useful, mechanical and electromagnetic properties. It has been reported that ZrO.sub.2 shows three polymorphisms such as cubic, tetragonal and monoclinic phases depending on the temperatures under ambient pressure.
Among various physical properties intrinsic to ZrO.sub.2, probably the most important is its phase transformability from a tetragonal to a monoclinic phase which is featured by a martensitic transformation. During the above phase transformation, there are neither heat emission nor absorption and no atomic diffusion, while a certain expansion of volume is observed. This property effects an important role to enhance the fracture toughness of polycrystalline ZrO.sub.2 or ZrO.sub.2 -reinforced composites. In particular, when a crack forms in a partially stabilized ZrO.sub.2 or a tetragonal ZrO.sub.2 polycrystal, the tetragonal ZrO.sub.2 around the crack tip is transformed to a monoclinic phase to increase its volume, whereby further development of the crack is prohibited. Similarly, dispersion of fine particles of ZrO.sub.2 in a ceramic matrix results in an increase in the fracture toughness of the composite. The elastic strain energy formed on the surface during the development of cleavage is reduced by the phase transformation of ZrO.sub.2 from a tetragonal to a monoclinic phase or by the action of the residual stress formed around the ZrO.sub.2 particles in the matrix.
Thus, it is important to control the phase transformation of ZrO.sub.2 in order to afford desired properties to polycrystalline ZrO.sub.2 or ZrO.sub.2 -reinforced composites. Accordingly, a number of extensive studies on this problem have heretofore been made. Such studies can be categorized into two types: one for controlling the particle size of ZrO.sub.2, and the other for changing the structural stability of ZrO.sub.2 itself by alloying ZrO.sub.2 with suitable solute atoms such as stabilizers or destabilizers (accelerants of the phase transformation).
Various oxides including Y.sub.2 O.sub.3, CaO and MgO have been known as the stabilizer to suppress the phase transformation of ZrO.sub.2 from tetragonal to monoclinic. However, no accelerants have been known for promoting the phase transformation except HfO.sub.2.
It has been reported by Claussen et al. that the transformation of a tetragonal phase of ZrO.sub.2 to a monoclinic phase can be accelerated by preparing a solid solution of ZrO.sub.2 with 30 to 60 mol % of HfO.sub.2 in an Al.sub.2 O.sub.3 matrix. [See, Claussen et al., Advances in Ceramics, vol. 3, p164, (1981).]However, this method has serious drawbacks that an exceedingly large amount of HfO.sub.2 should be used in order to attain the desired acceleration of phase transformation.
It has also been reported by Green that upon diffusing and extracting Y.sub.2 O.sub.3 over the surface of an Al.sub.2 O.sub.3 -(Y.sub.2 O.sub.3 doped) ZrO.sub.2 composite by heating the composite in the presence of pure zirconia, compressive residual stresses can be introduced on the surface of the composite, thereby resulting in an improvement in the surface strength. [See, D. J. Green, "A Technique for Introducing Surface Compression into Zirconia Ceramics," J. Am. Cer. Soc., 66[9], C-178, (1983).]However, this method has shortcomings that the amount of ZrO.sub.2 transformation from tetragonal to monoclinic is limited by the size of ZrO.sub.2 grains employed and that complicated and time consuming procedures should be carried out in order to remove Y.sub.2 O.sub.3.